This invention relates to an anion-deficient non-stoichiometric lithium iron phosphate as an electrode-active material, method for preparing the same, and electrochemical device using the same.
Lithium-ion secondary batteries have advantages of light weight and high capacity as compared with other secondary batteries such as nickel-cadmium batteries and nickel-hydrogen batteries. Lithium-ion secondary batteries are used as power sources of electronic instruments and appliances such as mobile phones, laptop computers, game devices, and wireless cleaners. Recently, their application field has been broadening to high-capacity batteries for electric bicycles, electric scooters, service robots, electric cars, and electricity storage devices for power plants.
Lithium-ion secondary batteries have generally been made by employing lithium cobaltate (LiCoO2) as a cathode-active material, carbon as an anode-active material, and lithium hexafluorophosphate (LiPF6) as an electrolyte. However, cobalt as the main component of lithium cobaltate (LiCoO2) suffers from unstable supply and demand, and its cost is high. Thus, it is economically unfeasible to apply cobalt to large-size lithium-ion secondary batteries which require a large amount of cobalt. Accordingly, other materials, such as lithium manganate (LiMn2O4) of spinel structure, where cobalt is substituted by other transition metals of low cost, began to be commercially used, and lithium transition metal polyanion compounds represented by lithium iron phosphate (LiFePO4) of olivine structure are also undergoing commercialization.
Lithium transition metal polyanion compounds of olivine structure have a stable crystal structure, are highly stable to chemical reactions, and have advantages of high capacity, long life, and low cost. However, their commercialization does not easily progress due to low electron conductivity, low ion conductivity, and by-products formed from side reactions. Among the drawbacks, low electron conductivity may be improved by coating conductive carbon, and low ion conductivity can be improved by shortening lithium ion diffusion routes by making particles ultrafine. However, the composition of a transition metal polyanion compound is prone to change depending on its preparation method, and it is difficult to obtain a compound having a desired composition and a desired oxidation state without impurities. Such impurities and undesirable oxidation states deteriorate the characteristics of the material and batteries, resulting in low productivity, low reliability, and low cost-effectiveness.
For example, during the preparation of LiFePO4, impurities such as Fe2O3, LiFeO2, and Fe2P are generated, and most of the iron contained in the impurities has a +3 oxidation state, because iron electrostructurally tends to have an oxidation state of +3 rather than +2, and iron can be easily oxidized during calcination.
If ferric ions (Fe3+) are present in lithium iron phosphate, the compound contains impurities which do not have an olivine structure and are not electrochemically active. Accordingly, a lithium iron phosphate containing iron of a +3 oxidation state is inferior to a lithium iron phosphate containing only iron of a +2 oxidation state in characteristics as a battery material. Therefore, efforts have been made to prevent the formation of iron of a +3 oxidation state.
The formation of iron of a +3 oxidation state is suppressed in the anion-deficient non-stoichiometric lithium iron phosphate of the present invention.
Although some non-stoichiometric lithium transition metal polyanion compounds are referred to in published literature, they are different from the compound of the present invention in that they are not anion-deficient compounds but cation-excessive or cation-deficient compounds.
For example, EP-A-1,094,532 describes a method for preparing a compound of the formula LixMyPO4, in which x is greater than 0 and less than or equal to 2, y is greater than or equal to 0.8 and less than or equal to 1.2 and M includes at least one metal having 3d orbitals.
EP-A-1,094,533 and Korean Patent Application Laid-open No. 2001-0025117 describe a compound represented by the general formula LixMyPO4, in which x is greater than 0 and less than or equal to 2, and y is greater than or equal to 0.8 and less than or equal to 1.2, with M containing a 3d transition state, and the grains of LixMyPO4 are no greater in size than 10 micrometers.
U.S. Patent Application Publication No. 2006/0263286A1 and Japanese Patent Application Laid-open No. 2006-131485 disclose a preparation method of olivine Li1+xFe1+yPO4, wherein −0.2≦x≦0.2 and −0.2≦y≦0.2.
U.S. Patent Application Publication No. 2007/0207080A1 discloses a method for preparing a LixMyPO4 compound having an olivine structure. The method includes: preparing a solution containing transition metal M ions, Li+ ions and PO43− ions; drying the solution to form particles of a starting material; and forming the particles of the starting material into particles of the LixMyPO4 compound with an olivine structure, in which 0.8≦x≦1.2 and 0.8≦y≦1.2, and coating the particles of the LixMyPO4 compound with a carbon layer thereon.
PCT Publication No. WO 2003/077335 and Korean Patent Application Laid-open No. 2004-0094762 disclose, as an electrode active material having the formula Aa+xMbP1−xSixO4 wherein (a) A is selected from the group consisting of Li, Na, K, and mixtures thereof, and 0<a<1.0 and 0≦x≦1; (b) M comprises one or more metals, comprising at least one metal which is capable of undergoing oxidation to a higher valence state, where 0<b≦2; and wherein M, a, b, and x are selected so as to maintain electroneutrality of said compound.
However, the lithium compounds referred to in the above literature are cation-deficient or cation-excessive compounds where the molar ratio of the anion like PO43− is fixed at 1 and the molar ratio of the cation M varies as in LixMyPO4, and thus are different from the anion-deficient compound of the present invention. The anion-deficient compound of the present invention is a compound represented by LixFe(PO4)1−y, wherein the molar ratio of the cation (i.e., iron ion) is 1 and the molar ratio of the anion PO43− is less than 1.
Non-stoichiometric lithium iron phosphate is a ceramic material which is an ionic compound. Non-stoichiometric ionic compounds comprising an anion (Xy−) and a cation (My+) may have four types of point defects, as follows: The first type is a cation-excessive non-stoichiometric compound represented by M1+zX. In this formula, the molar ratio of the anion (Xy−) is 1, which means that all the lattice points of the anion are filled. The molar ratio 1+z of the cation My+ means that all cation lattice points are filled and, in addition, the excessive z mol of the cation My+ are positioned at interstitial sites between lattice points. This type of point defect is called an interstitial cation defect. Here, if 1 mol of Xy− having a negative charge of −y is present and 1+z mol of the cation My+ having a positive charge of +y are present in excess of 1 mol, electroneutrality is not satisfied. Accordingly, the oxidation number of the cation cannot be maintained at +y but decreases to +y′(y′=y/(1+z)) to maintain electroneutrality.
The second type is a cation-deficient non-stoichiometric compound represented by M1−zX. In this formula, the molar ratio of the anion (Xy−) is 1, which means that all the lattice points of the anion are filled. The molar ratio 1−z of the cation My+ means that all cation lattice points are not filled but, instead, z mol of the cation lattice points remain as vacancies. This type of point defect is a cation vacancy defect. If the anion Xy− having a negative charge of −y exists in an amount of 1 mol and positive ions having a positive charge of +y exist in an amount of 1−z mol, which is less than 1 mol, electroneutrality is not satisfied. Accordingly, the oxidation number of the cation cannot be maintained at +y but increases to +y′(y′=y/(1−z)).
The third type is an anion-deficient non-stoichiometric compound represented by MX1−z. In this formula, the molar ratio of the cation (My+) is 1, which means that all the lattice points of the cation are filled. The molar ratio 1−z of the anion Xy− means that all anion lattice points are not filled but, instead, z mol of the anion lattice points remain as vacancies. This type of point defect is an anion vacancy defect. If the cation My+ having a positive charge of +y exists in an amount of 1 mol and the negative ions having the negative charge of −y exist in an amount of 1−z mol, which is less than 1 mol, electroneutrality is not satisfied. In ceramic material, cations are in general transition-metal cations and the oxidation number of a transition metal may vary in certain ranges but the oxidation number of an anion is hard to change. Accordingly, the oxidation number of the cation cannot be maintained at +y but reduces to +y′.
The fourth type is an anion-excessive non-stoichiometric compound represented by MX1+z. In this formula, the molar ratio of the cation (My+) is 1, which means that all the lattice points of the cation are filled. The molar ratio 1+z of the anion Xy− means that all anion lattice points are filled and, in addition, the excessive z mol of the anions are positioned at interstitial sites between lattice points. This type of point defect is an interstitial anion defect. If the cation My+ having a positive charge of +y are present in an amount of 1 mol and the negative ions having the negative charge of −y are present in an amount of 1+z mol, which is less than 1 mol, electroneutrality is not satisfied. Accordingly, the oxidation number of the cation cannot be maintained at +y but increases to +y′(Y′=y(1+z)).